1. Field of the Invention
The present invention relates to a rotational angle detecting device and detecting method that can detect a rotational angle of a wave generator of a wave gear device based on the output of strain detecting elements attached to the wave gear device.
2. Related Art
A wave gear device usually includes a ring-shaped rigid internal gear, a flexible external gear that is disposed inside the rigid internal gear, and a wave generator that has an oval outline. The flexible external gear is flexed into an elliptical shape by the wave generator, and engages the rigid internal gear at both ends of the major axis of the elliptical shape. As the wave generator is rotated by a motor or the like, the engagement positions of the both gears move in a circumferential direction, and due to a difference in the number of teeth between these gears, a decelerated rotational output is obtained from one of the gears.
A construction where multiple sets of strain gauges are stuck onto a diaphragm of the flexible external gear at intervals of a predetermined angle around a central axis thereof so that an error component due to cyclic displacement of the flexible external gear caused by the wave generator can be eradicated, is known as a torque sensor for a wave gear device. Based on the outputs of these sets of strain gauges, it is possible to eradicate error components (ripples in rotation) due to displacement of the flexible external gear that is not related to the transmission torque, so that the torque can be detected precisely.
Here, the flexible external gear flexed into an elliptical shape is repeatedly displaced in a radial direction at a rate of two cycles per single rotation. Accordingly, an error component that sinusoidally varies in two cycles per single rotation of a wave generator is included in an output of the strain gauge. The applicant of the present application has proposed the use of this error component in the detection of a rotational angle of the flexible external gear or the rigid internal gear that is an output member of a wave gear device in JP-A 10-19554.
The wave gear device is typically used in combination with an AC servo motor or the like to constitute a driving mechanism in an industrial robot, precision machining equipment, or the like. In those driving mechanisms, it is essential to detect the rotational angle of the motor shaft to control the driving of the motor.